In microelectronic applications, polymers that demonstrate high temperature resistance are generally well known. Precursors of such polymers, such as polyimides and polybenzoxazoles can be made photoreactive with suitable additives. The precursors are converted to the desired polymer by known techniques such as exposure to high temperatures. The polymer precursors are used to prepare protective layers, insulating layers, and relief structures of highly heat-resistant polymers.
Conventional positive-working photosensitive polybenzoxazole compositions contain an alkaline-soluble PBO precursor and a diazoquinone photoactive compound as disclosed in U.S. Pat. No. 4,371,685. The diazoquinone compound inhibits the solubility of PBO precursor in an aqueous base. However, after exposure to light, the diazoquinone compound undergoes photolysis and converts to indene carboxylic acid, which promotes solubility of the PBO precursor in the aqueous base.
Chemically amplified positive-working photosensitive polybenzoxazole compositions containing a PBO precursor, in which some of the phenol groups are protected with acetal protecting groups, and a compound which produces acid upon exposure to radiation (PAG) were disclosed in U.S. Pat. No. 6,143,467 and US Publication No. 20040253542 herein incorporated by reference. After irradiation with light of appropriate wavelength, the PAG produces acid, which removes the acetal protecting group from the phenol moiety. This process promotes dissolution of the PBO precursor in aqueous base to form images which have a reasonably high wall angle.
For some applications, microelectronic manufacturers and technologists request coating compositions that provide shallower or softer side wall geometries. If the sidewalls of the features that result from the photoimaging process are too vertical and/or form too sharp of an angle with the top surface of the coating, subsequent metallization of the features can lead to metal layers with high induced stress. Cracks and delamination of the metal layer may form in the regions of high stress leading to device failure. Further, too steep a sidewall can cause issues with some wire bonding equipment. The present invention discloses new photosensitive compositions comprising PBO precursors and a plasticizer that has a low vapor pressure at the typical soft bake temperatures (100-150° C.) for the film. The presence of these plasticizers in the formulation helps to provide lower side wall angles to reduce stress cracks and improve device reliability without detrimental effects on other lithographic performance aspects of the composition.